Yeah, stop rocking. Stay tight, and don't stop and look down at your feet between every rep. Keep your back tight hold your air and keep your head neutral or looking up slightly. You need to sit back more on to your heels, not down on to the balls of your feet.
Also, that weight was entirely too light. Its impossible to pinpoint your weak points watching you squat weight your grandma can squat. Post a near-max attempt so we can find your sticking point.